High frequency modulating and transmitting system



e. T. RoYDE-M 2,165,112

RAN-SMITTING SYSTEM Filed Jam. 7, 1958 July 4, 1939;

HIGH FREQUENCY MODULA'IING AND in l CARRIER GENERATOR m .Y m a T R mmm l n [RU L T I N wm m m v T E a T m w A 6 7 UR EN T $00,205

+ P ATE 45 ciently Patented July 4, 1939 UNITED STATES PATENT OFFICE 2,165.1r2 I HIGH REQUENGY MODULAT'ING Ann N T NG. Y E 1 George T. Royden, South 'O'rang e, N; J.',- 'as'signor to Federal Telegraph Company," Newark, N. 'J., a corporation of New Jersey Application January .1, 13:38. serials; 183,773

6-Claims. (Cl. 179"17 1) The present invention relates to high frequency transmitting systems wherein a high frequency carrier is modulated in accordance with high frequency signals and then transmitted to 5 an antenna or transmission line. More ,particu larly the invention relates to a system for. pro viding a carrier'of sufiicient amplitude fortransmission to a remote point modulated faithfully in accordance with a high frequency signal.

it) The invention is particularly applicable to television transmission systems. In such systems it is usual to generate the high frequency carrier and amplify this carrier in a chain of sue} cessive amplification stages before transmitting 15 it to the transmission line or radiating antenna amplifiers subsequent to 'the m du Stage shall have very'linear characteristics, and for the wide band of frequencies required for television such linearity is difiicult to attain.'

It is therefore an objectof the present invention to provide a television transmitting ,system of the type wherein a carrier is generated and amplified in a chain of successive amplification stages before delivery to a transmission line or antenna for transmission to a remote point, but in which modulation of this carrier is effected in the last stage, or one of the later stages of the amplification chain.

It is also another object of the present inven- 40 tion to provide a novel modulation circuit particularly adapted for performing the modulation in the above mentioned television transmitting system. More generally it is an object to provide a modulation system which will emand faithfully modulate a high-power, high-frequency carrier in accordance with. a modulating signal which is itself of high frequency.

It is a particular object of the present inven- 5 tion to provide a modulating'system having a modulating tube and a driver tube arranged; in series, which is suitable for modulating .a highpower, high-frequency carrier .inaccordance with a high-frequency signal in a faithful and 55 linear fashion so that the envelope of the modu- "latedH cari'ier.iconstitutes a high fidelity represeritation o'f"thel w'aves formed of the high-frequericy' r'nodulati'ng signals. More especially it isan "object of the present invention to provide such an odulation system in which distortions dueto overloading of the modulating tube by stray. capacity effects are eliminated by a shield maintained at a varying potential.

' Thefinvention' can best be understood by refi rence theattached sheet of drawings, in which 19 Fig, lrepresents a modulating and transmitting"fs tern for television in accordance withthe present invention;

Fig. Zirepresents those parts of the series modulatio arrangement of Fig.1 which are .essentialf forfunderstanding the principles of the 1 'yarying potential shield of my invention;

worksiused for analysis of the circuits of Figs. 1

and 2 at' the'frequency of the modulating signals.

g more particularly to Fig. 1, a carrier; freque' cy oscillation generator I is connected't'o supply a carrier frequency to a radif" Referri ating antenna 2 by way'of a chain of amplifiers "3, 'a' r'nodulating tube 4 and a transmission line vision" scanner 12" supplies modulating signals through signal amplifier 13 to a pair of driver 5 tubes 14 and 15. Driver tube l4 serves to apply the amplified modulating signals to the modulatingutubej, thesetwo tubes 4 and I4 being actually connected in series with each other withrespect'to plate current source I6, so that 40 variations in the resistance of driving tube [4 "cause corresponding variations in the effective platepotential of modulating tube 4. The other .gdrivertube 15 is provided for a purpose more vfully explain'edhereaften. the modulating signals developed in de- Vice [2 and amplified in device 13 serve to vary the internalresistance of driver tube M which in turn varies the effective plate voltage of modulating tube 4'. As a'result of this variation in its effective 'plate voltage, modulating tube 4 controls the amplitude of the carrier transmitted ,fronramplifier chain 3 in a faithful, substantiallylinear manner'so'that the envelope of the carrier delivered through coupling unit In to the 5 transmission line 5 corresponds accurately to the variations of the effective plate voltage of the modulating tube 4. r

The cathode of the modulating tube 4 which is preferably of filamentary type as shown, is energized from a suitable source of alternating power through a filament transformer which comprises ,a primary I1 and'a; secondary 18 adapted to be shielded from one another electrostatically as more fully explained hereafter. A grid leak l9 together with a pair of by-pass condensers 20 and 2| are'connected in.

the grid return circuit of the modulating tube 4 as shown, to serve as biasing means. for the grid. I of this modulating tube This leak and pair of.

condensers I9, 20 and 2| act in similar manner to the well known grid leak and condenser ordinarily' employed, but the connection shown as somewhat superior to the usual connection from the standpoint of balance. If the distributed capacity of primary I7 is not sufli'cient to by-pass the plate" of drivertube l4 to the filament of modulating tube..4, further condense'rsrmay be 7 2!, as well as the secondary 8 and condenser 9 shield all the elements enclosed within it from of the tuned coupling unit 6 previously described. This shield 22is disposed soas to electrostatically external portions of the circuitand from ground. By ground is meant either the actual earth or the surrounding environment; Thus if the equipment is housed in a cabinet the potential ofthis cabinet is to be considered as ground potential, while if the equipment is mounted on a chassis but not housed the average potential or. the chassis and other surroundings is to be taken as ground potential. It is desirable also to reduce the capacity between shield 22 and ground'as far as is conveniently possible and for this purpose all the elements shown in Fig. 1 as beingwithin the shield are preferably grouped together as closely as possible and arranged so that the shield can readily enclose them all without being of excessive size. 7

r The shield 22 is connected to the second driver tube 15' so that the potential of this shield will becontrolled by this second driver tube. Preferably the shield is directly connected to the anode of the driver tube l5 as shown, and a load impedance 2 3 is interposed between the plate of. this tube andthe power source Hifso as to regulate the phase and amplitude of the potential variation of the'shield'22- The second drivertube I 5' is fed in parallelwiththe' first'driventube H from the amplifier 3, as shown in Fig. 1, and the load impedance 23 is so proportioned with respect to the effective impedance of shield 22, that the'potential'of shield 22 will vary in substantially the s'ame manner as the potential of the 7 plate of driver tube H. The function of this shieldv 22"whose potential varies as above'described, is to reduce ,the effective variation of onnected to the anode of driver tube I4 with respect to'mrr's'up frequency waves, the signal As can be .seen "perhaps roundings, so that the charging current drawn by all these elements will be reduced so as not to overload the driver tube I4. The significance of this arrangement can best be understood from a consideration of Figs. 2, 3 and 4.

Fig.2 schematically represents sufiicient of the series "modulating connectionsof tubes I4 and 4 to enable. thefunctiongof the shield 22 of Fig. 1,

to be clearly understood. As is clearly evident in Fig. 2, all the elements 8, 9, l1, l9, 2|], 2| as well as the-filament and grid of modulating tube 4 and the anode ofdriver tube l4 and all the wiring interconnecting the above named parts, may convenien'tly' be considered as one element with re spect to the frequency of the modulating signals since for this signal frequency the impedance of the tuned circuits 1 and 8 will presumably be negligible. All these'parts and their interconnecting wiring, therefore, may most conveniently be. considered as one element having at any one instant one potential so far as the signal 'frequencywaves are concerned, and'for convenience allv these parts and their wiring have been shown dotted in Fig. 2 and are collectively designated as Y. ffIhe capacitance of all these elements with respectjjto ground may bev collectively designated as Cy and may be approximately represented'by a comparatively large condenser as shown in Fig.'3.'-'

The anode X,.of' modulating tube .4'may also be considered to. be directly connected to ground since the impedance of coupling device-l 0 maybe assumed to'be negligible atthe frequency of the modulating signal, and since the plate current U source l6 may be assumed to have negligible impedance at this frequency if it is properly bypassedg The'coll'ective capacities of all the parts generally designated Y with respect to the anode X 'of modulatingftube 4 may fbe collectively designated'Cxy and may be approximately represented by a condenser as in Fig. '3. The cathode of driver tube l4 may also be considered as directly connected to ground and as having acapacity with respect to all the parts Y.

The grid of this modulating tube l4 cannot accuratelybe consideredffas 'connected'to the cathode of the tube l4 beeause a very considerable between these two electrodes;

impedance exists Nevertheless, by

takingaccount of the variations of potential of the grid oftube I4 thefgrid and'cathode of the tubev l4 may be grouped togetheras Z, and for any given potential variations ofY and of the grid the capacitybetween Y and Z may; be approximately represented by a condenserCyz as in Figs.j3 and 4. The internalresistances of tubes l4 and 4 may be approximately represented as resistors R14 and R4 as-shown in Fig; 3.

Referring to the schematic diagram of Fig. 3, which, approximately represents the impedance relations of the circuit of Fig. 2 for the signal veloped in tube l4 is repr'esented as developed frequency energy ,de

by a generator V14 in series with a resistor R14.

is prevented by means of the shield 22shown in Fig. 1 and Fig. 4, which effectively splits the condenser Cy shown in Fig. 3 into two condensers Cys most clearly in Fig. 3, j

and Cs connected in series as shown in'Fig..4. The condenser Cys represents thecapacity between all of the parts Y and the shield 22 while the other condenser Cs represents the capacity of the shield 22 to ground. To the intermediate point between these twocondensers is then connected the signal frequency energy developed in auxiliary tube 15. This energy is represented in Fig. 4 as developed by generator V15 in series with resistor R15 and shunted by load impedance 23 of suitable value.- As far as possible the load impedance 23 is adjusted so as to cause shield 22 to vary in potential in nearly the same manner as all of the parts collectively designated as Y. By this expedient the charging current required for charging the parts Y'is greatly reduced so that the driver tube I4 is enabled to operate substantially without distortion.

By this arrangement the efiiciency and highpower linear modulation qualities of the series modulation circuit are attained and at the same time the overloading of the driver tube I4 is obviated. Thus the total modulation system is enabled to operate at a high eificiency and with practically no distortion, and is capable of modulating a carrier of sufficiently high power so that this carrier may be directly employed for transmission to a remote point without further amplification or with only a small amount of further amplification.

Although it is preferred to enclose all of the parts collectively designated as Y within the floating shield 22, it is possible to attain many of the advantages of the present invention by enclosing only certain ones of these parts within the shield. It is possible, for example, to arrange the shield so that the driver tube I4 is wholly or partially outside of this shield, although in such a case there will be an appreciable capacity be tween the anode of the driver tube and ground, which capacity will not be compensated by the shield. Also it should be understood that the floating shield need not be fed by an auxiliary tube connected in parallel with the driver tube H, but may be otherwise energized provided that the potential of this shield is varied approximately in accordance with the potential of the parts collectively designated as Y, so as to reduce the charging current, ground, to driver tube Hi.

In the preferred embodiment above described and illustrated, the series connection of the driver and modulator tubes is effected by connecting the modulator tube on the positive side of the driver tube with the cathode of the latter tube grounded. Some of the advantages of the invention may be attained, however, with the relative positions of these two tubes interchanged and/or with some other point in the circuit grounded. Also the output circuit Hi may be disposed in a different position although the disclosed arrangement is preferred.

Although I have shown and disclosed certain embodiments of my invention for the purpose of illustration, it will be understood that modifications, adaptations and variations thereof occurring to one skilled in the art may be made without departing from the scope of my invention as defined in the appended claims.

What I claim is:

1. A television modulation system for modulating a high frequency, high power carrier in accordance with high frequency signals which comprises a modulator space discharge device comprises a modulator space discharge path, a

drivhaving a modulating space driver space'idischarge device having a ing space discharge path, a current source, means for connecting the space discharge paths in series with each other and said source, means for applying said carrier to said modulating path, means for varying the impedance of said driving path in accordance with said signals for effecting modulation of said carrier, shielding means for dividing the effective capacity across said driver discharge path into two serially related capacities, and means for varying the potential across one of said two capacities substantially in accordance with variations of potential across said driver discharge path at least with respect to said signals.

2. A television modulation system for modulating a high frequency, high power carrier in accordance with high frequency signal which discharge device having a modulator anode and a modulator cathode and a modulating space discharge path therebetween, a driver space discharge device having a driver anode and a driver cathode and a driving space discharge path therebetween, a

current source, means for connecting said driver anode to said modulator cathode and said driver cathode and modulator anode anode to said source, connections between said current source and ground, means for applying said carrier to said modulating path, means for varying the impedance of said driving path in accordance with said signals for effecting modulation of said carrier, shielding means for dividing the effective capacity between said modulator cathode and ground into a first capacity between said modulator cathode and said shield and a second capacity between said shield and ground, and means for varying the potential of said shield substantially in accordance with potential variations of said modulator cathode at least with respect to said signals.

3. A television modulation system for modulating a high frequency, high power carrier in accordance with high frequency signals which comprises a modulator space discharge device having a modulator anode and a modulator cathode and a modulating space discharge path therebetween, a driver space discharge device having a driver anode and a driver cathode and a driving space discharge path therebetween, a current source, means for connecting said driver anode to said modulator cathode and said driver cathode and modulator anode to said source, connections between said current source and ground, other appartus having a considerable distributed capacity to ground, connections between said modulator cathode and said other apparatus, means for applying said carrier to said modulating path, means for varying the impedance of said driving path responsive to said signals so as to vary the potential of said driver anode and modulator cathode in accordance with said signals for efiecting modulation of said carrier, a shield around said other apparatus, and means for varying the potential of said shield substantially in accordance with the potential of said driver anode thereby reducing the eifective capacity between said other apparatus and ground with respect to said signals.

4. A television modulation system for modulating a high frequency, high power carrier in accordance with high frequency signals which comprises a modulator space discharge device having a modulator anode and a modulator cathode and a modulating space discharge path therebetween, a driver space discharge device havinglaedriveranode and azidriver cathode-and a driving'space discharge :path therebetween, a current source, meansvfor connecting said driver anode to said modulator cathode and said driver cathode and modulator anode to said source, connections between said current source and ground, other apparatus having a considerable distributed capacity to ground, connections between said modulator cathode and vsaid other appartus, means for applying said carrier to said modulating path, means for varying'the impedance of said driving path responsive to said signals so as to vary the potential of said driver anode and modulator cathode in accordance with said signalslfor effecting modulation :of. saidcar- V .rier,.a shield around said ;other apparatus,.a.further space discharge device having a i-further anode and a :further :cathode. and'lalfurtherv space discharge path ltherebetween, means for varying the impedance of said further .path in accordance with said signals, and means ,for :varying vthe potential ofsaidshield responsive to the'impedance variations 'of said. further path in sub- ,stantially the .samephaseland amplitudeas the potential of said driver .anode thereby reducing the-efiective capacityibetween said other apparatus and;ground: with respectto said signals.

'A carrier signalling system comprising a tionofesaid tubes and'the cathode of said secondtube, atshieldt'around said-means, means for applying a carrier frequency to said first tube, and means for simultaneously varying the impedancesaof said secondtube and'the potential of :said;shie1d:to reduce said undesired capacity effeet.

6. A system accordingto claim5 wherein said means. forisimultaneousl-y varying theimpedance of said second tube and the potential of said shield comprises a third discharge tube having its anode connected to said shield and its cathode connected to the cathodeof said second tube,and common means for varying .the impedances of said,second andthird tubes in the same phase relation. v s

GEORGE T. ROYDEN. 

